1. Field of the Invention
The present invention relates to a brush type charger, and more particularly to a brush type charger installed, for use, in an image forming apparatus such as a printer, a facsimile or a copying apparatus using an electrophotographic process.
2. Description of the Prior Art
Conventionally, in a process cartridge for use with this sort of electrophotographic process, there has been utilized a non-contact type corona charger using corona discharge as a charger for charging a charging object prior to formation of an electrostatic latent image.
The corona charger is effective as means for uniformly charging a charging object, but simultaneously has a problem that it causes ozone to occur with corona discharge.
Since it is capable of restraining occurrence of ozone in contrast to the non-contact type corona charger, a contact type charger has been put to practical use in many apparatuses in recent years.
Of the contact type chargers, when DC voltage is applied to a brush type charger, brush marks-shaped density unevenness caused by a brush occurs on an image. This brush marks-shaped density unevenness can be avoided by superposing AC component on DC voltage as applied voltage, and at the same time such a stable charging that is not affected by any environmental fluctuation, becomes possible.
According to the content of technical contents disclosed in, for example, Japanese Patent Application Laid-Open Nos. 60-220587 and 7-98534, it is disclosed that uniform charging is performed by applying voltage having AC voltage of high frequency wave superimposed on DC voltage to a brush charging device to provide an excellent image.
In the technique disclosed in the Japanese Patent Application Laid-Open No. 60-220587, however, noise occurs because an AC component of high frequency wave exceeding 500 Hz is superimposed.
Also, the technique disclosed in the Japanese Patent Application Laid-Open No. 7-98534 is an example applicable to a non-rotary type brush charging device, and therefore, it is difficult to maintain an excellent image over a long term, and the problem of noise also remains unsolved.
Further, as another known example, there is Japanese Patent Application Laid-Open No. 8-137187. In the technique disclosed in the Japanese Patent Laid-Open Application No. 8-137187, the frequency of AC voltage to be applied to a rotary type brush charging device is represented in terms of relation between process speed and brush pile pitch. The rotary type brush charging device has an advantage of being able to maintain an excellent image over a long term.